The European Space Agency – of which, for the time being, the UK remains a fully engaged member – has quietly cleared for takeoff a space mission to address the biggest question of all: what is the universe made of? Galaxies, stars, black holes, asteroids, planets and people together add up only to a trifling 4% of all that there is: the remaining 96% is mysterious and very dark. The agency's Euclid is an optical and infrared space telescope that will be launched in 2020, to spend six years a million miles beyond Earth, measuring with subtle techniques and exquisite precision the geometry, distribution and acceleration of billions of galaxies across distances that extend 10bn years back in time.

Modern observational science began when Galileo turned a rudimentary pair of lenses on the moon and Jupiter. The paradox is that each great advance since then has successively also exposed even bigger questions about the firmament above and the emptiness around us. It was only in the 1960s that radio astronomers confirmed that spacetime, radiation and atomic matter all had their origins in a big bang less than 20bn years ago. But even before this, observers had begun to puzzle about the behaviour of the galaxies: none of them seemed to have anything like the gravitational mass implied by their shape and structure.

That was the point at which physicists began to propose a mysterious component of the universe called dark matter. This strange stuff does not shine or glow, does not bounce off anything or announce its existence in any recognised way. Nevertheless it has gravitational mass that glues an estimated 200bn galaxies, each of perhaps 200bn stars, into cohesive and enduring structures. And, clearly, it far outweighs all visible matter. Only 14 years ago, physicists contemplating the most distant galactic supernovae made the Nobel prizewinning discovery that these distant brightnesses were receding at an ever-faster rate, when the logic of gravitational theory suggested they should be slowing down. It was as if some kind of antigravity, detectable only at vast distances, and accounting for an estimated 73% of the total mass-energy of the cosmos, had taken charge, and would eventually disperse all other galaxies beyond the universal horizon, condemning any survivors in a far-distant future to an eternity of frozen blackness.

The Euclid project is a partnership of 1,000 scientists from 100 institutes. Their instruments will make the most meticulous measurements of galactic behaviour, on the principle that riddles that have been revealed by careful examination may logically be solved by even more careful examination. What's the betting, though, that even if it does deliver a compelling answer, Euclid will also unveil an even more astonishing set of questions?